The proposed study is a continuation of a follow-up study on a sample of 64,185 of the 280,000 women who previously participated in a five-year multi-center breast screening program, the Breast Cancer Detection Demonstration Project, conducted during 1973-980. Continued follow-up of this cohort will be valuable both with respect to issues of prevention (e.g., survival of screening-identified cases, persistence of use of screening, identification of very of very high-risk groups for screening) and etiology. The value in continuing to follow the cohort lies in the opportunity to address prospectively a number of hypotheses in cancer etiology having direct relevance to prevention strategies, taking advantage of the substantial amount of relevant data and materials already collected on this population of women. Continuation of the follow-up will allow accrual of additional cases of breast cancer and other diseases, thus increasing the overall power of the study. One area for which this is particularly important is in assessing the risks and benefits of menopausal hormone replacement therapy. Continuation of the study will also be essential to effectively utilize the risk factor data that were collected only in the 1993/1995 questionnaire. This includes eating habits during the past year, periods of weight gain and loss, and distribution of body weight.